Following the revelations of General Petraeus' affair, it had emerged that Ms Kelley, a friend of the Petraeus family, had complained of allegedly harassing emails from Ms Broadwell.

A US official said the FBI had uncovered between 20,000 and 30,000 pages of communications - mostly emails and spanning from 2010 to 2012 - between General Allen and Ms Kelley, from Tampa, Florida, who is a volunteer social liaison with military families at MacDill air force base.

While the matter is under investigation and before the facts are determined, General Allen will remain commander of Isaf. Leon Panetta

US Defence Secretary Leon Panetta confirmed that he had referred "a matter involving" General Allen (on the right of the above picture with General David Petraeus) to the Pentagon.

Asked whether there was concern about the disclosure of classified information, the official, on condition of anonymity, said: "We are concerned about inappropriate communications. We are not going to speculate as to what is contained in these documents."

Scandal

General Allen, who is 58 and married, had been nominated by US President Barack Obama to be supreme allied commander Europe and commander of the US European command.

Although given the circumstances, that nomination has been deferred, General Allen will continue to command US forces and Isaf in Afghanistan according to a US government source.

National Security Council spokesman Tommy Vietor urged the Senate to "act swiftly" on his nomination of General Joseph Dunford to succeed Allen as commander in Afghanistan.

Leon Panetta said that while the matter is being investigated by the Defence Department Inspector General, John Allen will remain in his post based in Kabul. Mr Panetta praised Allen as having been instrumental in making progress in the war.

The US defence official said that Allen denied any wrongdoing and that Panetta had opted to keep him in his job while the matter was under review, and until Dunford can be confirmed to replace him - a process that gains urgency given the potentially lengthy review process and the cloud it could cast over the mission in Afghanistan.

"While the matter is under investigation and before the facts are determined, General Allen will remain commander of Isaf," Panetta said, referring to the Nato-led force in Afghanistan

General Allen took over in Afghanistan because of a scandal involving his predecessor.

The former leader of the International Security Assistance Force (Isaf) in Afghanistan, Major General Peter Fuller, was relieved of his duties after making remarks which were critical of Afghan President Hamid Karzai.